1. Field of the Invention
This invention relates to an acceleration/deceleration control apparatus for controlling the acceleration/deceleration of a controlled object having a plurality of drive shafts of a tool or the like, such as a numerically controlled machine tool.
2. Description of the Related Art
In an ordinary machine tool, it is required that the velocity at which a tool is moved be set at an optimum value in dependence upon the material machined and the type of machining. In a case where drive shafts are controlled by a numerical control unit, the target position and velocity of a tool under control are commanded simultaneously from a command tape. In order to smoothen out the motion of the tool, distributed pulses for each axis are revised by an acceleration/deceleration circuit and the feed rate along each axis is decided.
When acceleration/deceleration is applied to a commanded velocity, accumulation occurs along each axis with regard to the commanded position of the controlled object, and a delay is generally produced at the control position. Since the amount of accumulation differs for each axis, a deviation in the trajectory of movement occurs at corner portions, where machining precision is required. In particular, when there is a gap in commanded velocity between consecutive command data blocks, the acceleration/deceleration control circuit is so arranged that the controlled object is stopped repeatedly and the next item of command data is read out after the object is stopped each time, thereby eliminating the accumulation along each axis.
If the above-mentioned deviation in the trajectory of movement occurs in such a conventional acceleration/deceleration apparatus, then when a transition is made from a command data block in which machining velocity rises with an increase in machining distance to command data block in which machining velocity falls with a decrease in machining distance in, say, a cutting operation. As a result, there is a greater amount of sag in the machined shape.
In particular, in a machining operation in which a machined shape 20, which is the goal of machining, has a protruding portion 20a, as shown in FIG. 4, a cutting tool (cutter) is controlled for back-and-forth machining. However, despite the fact that the tool is controlled by the same machining commands, there are occasions in which a deviation occurs between a machining path 21r from the right of the diagram and a machining path 21l from the left of the diagram. This deviation in machining trajectory, which is caused when machining a groove or a protrusion by back-and-forth machining, gives rise to a delay in response to command data on the servo mechanism side. In order to eliminate this deviation in trajectory, it is required that the velocity of cutting tool movement be sufficiently reduced in dependence upon the precision demanded by the object of machining.
More specifically, if a difference in level caused by a deviation in machining trajectory in back-and-forth machining is to be reduced or completely eliminated, as shown at point P in FIG. 4, a transition to the next block of processing must be made upon confirming stopping of the tool in every block of command data. With such conventional acceleration/deceleration control of a machine tool based upon this method of reading out command data, a problem that arises is a deterioration in machining efficiency.